Highly Efficient Water Molecule Heating Technology Based on Semiconductor Multilayer

Water and most organic solvents have light absorption originating in the 3 μm band. For the realization of ideal heating system, optical bandpass filters are one of the efficient ways to transmit light having specific wavelength^[1]. Optical band pass filters are realized by dielectric multilayers consisting of materials with different reflectance, such as SiO2/TiO2. However, they are thermally unstable^[2], and further number of layers are required to achieve advanced performance. In this study, we propose a corundum-structured semiconductor multilayer film based on α-Fe₂O₃ (n=2.79^[3]) and α-Ga2O3 (n=1.74^[4]). Its small lattice mismatch (1.3%) and high crystallinity has the potential to solve problems of conventional dielectric multilayers. Furthermore, we design a new semiconductor multilayer mirror by them, which is expected to significantly improve the energy efficiency of water heating system. This mirror is designed to transmit in the 3 µm band (water absorption wavelength) and reflect in other wavelengths. Our heating system uses the light reflected by this mirror to heat the blackbody. The heated blackbody radiates containing 3 µm band again, so that light energy that does not originally contribute directly to heating can be re-used.<br/>In this study, a designed multilayer film is a stacked structure of α-Fe₂O₃ and α-Ga₂O₃, with a total of six layers. The calculated optical property is that transmits in the 3 µm band and reflects in the 2 µm and 4 µm bands, and the energy efficiency gains are 30% compared with a non-mirror heating. Furthermore, the refractive index and extinction coefficient, which are necessary for the design, were measured in our group. This is because they have rarely been measured for α-Fe₂O₃ and α-Ga₂O₃ thin films prepared by mist CVD. The multilayer was deposited by mist CVD, which is suitable for this study because it is simple and allows the deposition of oxide semiconductors with very high crystallinity. In most cases, c-plane sapphire substrates are superior in terms of crystallographic orientation. However, in this report, m-plane sapphire substrates were used to suppress κ-phase mixing, which is a factor in the deterioration of surface flatness over large areas. α-Fe₂O₃ was grown at 500°C and α-Ga₂O₃ at 475°C. Therefore, thermal stability of at least 500°C or lower is expected.<br/>First, α-Fe₂O₃/α-Ga₂O₃/sapphire interface was observed by TEM for microstructure analysis of interface. The results showed that each interface was atomically flat. Furthermore, STEM-EDS showed each element uniformly distributed. SEM observations, which were crack-free over a wide area, also reinforce that α-Fe₂O₃ and α-Ga₂O₃ are suitable for this mirror.<br/>The results of the two-layers film was satisfactory, and six-layers film was produced based on this results. The difference in thickness of each layer of the designed multilayer film and produced film was at most less than 7% by cross-sectional SEM. With this design, assuming a homogeneous film, a layer thickness error of less than 10% has little effect on the optical properties. Regarding the measurement results of optical properties, the transmittance in the 3 μm band was as high as the design value (80% or more), and the 2 μm and 4 μm bands also showed characteristic reflectance properties. In addition, there was no strong absorption in the near- and mid-infrared regions, and the absorption wavelength in the far-infrared region was almost consistent with the designed value. The improvement in energy utilization efficiency calculated from these measurements was about 24%, suggesting that multilayer films with new applications and functions have been realized using oxide semiconductors with a corundum structure.<br/><br/>[1] S. Jun et al., Drying Technol, 21, 51 (2003). [2] L. Qrlovskiy et al., Fusion. Eng. Des., 86, 1290 (2011). [3] W. E. S. W. A. Rashid et al., IEEE Access, 8, 105156 (2020). [4] H. He rt al., Phys. Rev. B, 74, 195123 (2006).